The present invention relates generally to internal combustion engines, and particularly to two-stroke and four-stroke engines having an improved lubrication system capable of reducing polluting emissions.
The present invention recognizes the global need for reduced hydrocarbon emissions from small power-producing engines, especially as relates to the rapidly growing demand for agricultural and light industrial power in developing economies. In these economies, the low weight and low cost of two-stroke engines will be difficult to ignore, and it may be expected that two-stroke engines will be widely used. Two-stroke engines produce high levels of unburned hydrocarbon emissions, since, due to their operating principle, exhaust gases are expelled from the engine""s cylinder at the same time that a fresh fuel/air charge is brought in, leading inevitably to mixing between the two and the inadvertent expulsion of unburned charge with the exhaust gases.
Furthermore, two-stroke engines pass their fuel/air charge through the crankcase to allow a slight pressurization of the charge, caused by the descent of the piston, to assist the flow of charge into the cylinder. As it passes through the crankcase, the charge entrains lubricating oil droplets, which are splashed on the crankshaft main bearing and connection rod (crank) bearing and sprayed on the cylinder walls and wrist pin. Alternately, oil is mixed with the fresh charge before entering the crankcase, in which case the charge is used as an agent for transporting oil to the engine surfaces requiring lubrication. Lubricating oil entrained in the charge is inducted into the cylinder, where it either flows through into the exhaust, creating more unburned hydrocarbon emission, or remains in the cylinder where it is burned, creating a more noxious set of pollutants than would stem from the combustion of the engine fuel itself.
The pollution disadvantages of conventional two-stroke, spark-ignited engines (overlap of intake and exhaust flows and crankcase charge compression) lead to its advantages in day-to-day applications. Since the exhaust and intake strokes are not separate, for a given requirement for engine power and speed, at a gas constant compression ratio, a two-stroke engine requires only half the displacement of a four-stroke engine. The weight of the two-stroke engine is also a little more than half of the weight of a power-equivalent four-stroke engine and cost much less to produce. These advantages will prove very difficult to ignore in a developing economy, and thus, if two-stroke engines retain their conventional form, there is a great potential for globally significant increases in engine-related air pollution.
The present invention retains the engine size advantage of the two-stroke engine, the cost advantage of the carbureted two-stroke engine and reduces its unburned hydrocarbon emissions and lubricating oil combustion characteristics to levels comparable with the most advanced direct injected, two-stroke, dry-sump engines. This is accomplished with a relatively minor increase in cost for the inclusion of new parts and new machined or cast features on conventional parts. These parts and features provide an improved two-stroke, spark-ignited engine capable of operating with very little unburned fuel emission and with very little lubricating oil combustion. The present invention is also applicable to four-stroke spark-ignition engines and compression-ignition engines such as diesel engines.
Nearly complete reduction in lubricating oil combustion in two-stroke engines is achieved by the present invention by using a novel system for dry-sump lubrication. The novel lubrication system is also applicable to four-stroke engines and provides various advantages, including the ability to operate the engine in any attitude or orientation and can provide supercharging at little added cost. In accordance with the present invention, the piston is provided with upper and lower seals defining an annular oil chamber in cooperation with the body of the piston and an adjacent portion of the cylinder wall as the piston is reciprocated within the cylinder. If necessary, an oil sleeve may be added between the cylinder and crankcase to effectively extend the cylinder wall to ensure that an annular oil chamber is defined along the desired length of the piston""s stroke.
The lower seal substantially prevents any oil within the annular oil chamber from flowing into the crankcase. Optionally, a small, controlled amount of oil is allowed to escape past the upper seal, into the upper portion of the cylinder, in order to lubricate the compression rings and then be consumed, as is normal practice in engine design. The remainder of the oil is circulated through the annular oil chamber to lubricate the cylinder, piston, compression and/or oil control rings, and/or seals. Depending upon the position of the seals, the annular oil chamber may also lubricate the wrist pin. Where desired, oil conduits or passages in the body of the piston may connect the wrist pin area with the oil chamber. Optionally, a system of sealed passages or conduits leading to and/or from the annular oil chamber may be provided to lubricate bearings in the crankcase. Oil from a reservoir is circulated through the annular oil chamber and/or the conduits by a pump. Because the oil reservoir is segregated from the crankcase by seals, the crankcase remains dry.
In its preferred embodiment, the invention concerns an internal combustion engine having a piston reciprocable within a bore of a cylinder. The piston is pivotally connected to a crankshaft by a piston rod having at one end a wrist pin engaging the piston and at an opposite end a crank bearing engaging a throw of the crankshaft. The crankshaft is rotatably mounted on a main bearing within a crankcase positioned beneath the cylinder bore.
A first seal is mounted on and circumferentially around the piston to define an upper end of the annular oil chamber. The first seal has an outer circumference engaging the cylinder to limit any oil flowing from the annular oil chamber to the cylinder bore for lubricating the cylinder.
A second seal is mounted on and circumferentially around the piston to define a lower end of the annular oil chamber. The second seal has an outer circumference engaging the cylinder to substantially prevent any oil within said annular oil chamber from flowing into the crankcase.
Upon motion of the piston within the cylinder bore, the first and second seals and the annular oil chamber move with the piston. At least a portion of any oil within the annular oil chamber is thereby carried with the piston to lubricate the cylinder, piston, compression and/or oil control rings, and/or seals.
In a conventional two-stroke engine, oil is either broadcast as a spray throughout the crankcase or inducted as a mist with the charge air. In both cases, the lubrication points are serviced by filling the entire crankcase with oil droplets. Many of these are inevitably inducted into the cylinder. In a two-stroke engine including a lubrication system according to the present invention, oil is selectively distributed to surfaces where it is needed for lubrication, and oil droplets do not enter the charge air stream. Therefore, lubricating oil consumption is limited to small amounts spread on the cylinder walls and seeping through the piston ring gaps, as is typical of a four-stroke engine. The lubrication system of the invention greatly reduces the excessive oil combustion and unburned emission of conventional two-stroke engines (especially at idle speeds), which has reduced two-stroke acceptance on environmental grounds. The invention""s lubrication system makes the task of premixing oil and fuel unnecessary and avoids the loss of lubricating potential attendant to dilution with fuel. Employment of the invention should lead to a reduction in lubricating oil consumption, thereby lowering the operating cost of such engines. The lubricating system also reduces spark plug fouling and combustion chamber carbon deposits, because very little lubricating oil is burned in the cylinder. The reduction in oil consumption in the cylinder inherent in dry-sump lubrication might make it feasible to equip the a two-stroke engine according to the present invention with a catalytic converter. Catalytic converters are not presently used on conventional two-stroke engines because they become fouled with oil emitted from the cylinder.
Optionally, the present invention may be combined with features, including: (1) separate scavenging and charging air flows; (2) a throttleable charging air flow; (3) a port opening sequence wherein the exhaust port opens, followed by the scavenging port opening, followed by a charging port opening; and (4) variable exhaust port timing; as disclosed in commonly owned U.S. Pat. No. 6,397,795, the entire disclosure of which is hereby incorporated herein by reference.